Once a star has used the core's hydrogen and converted it to
helium, it can no create energy that way, and must use other resources. This
point is the trigger for the advanced evolution of the star, and over the next
few billion years, it will evolve to a red giant star, and then decay into a
planetary nebula.

Red Giant Ascent

After about ten billion years, a main sequence star has converted
approximately 10% of its hydrogen to helium. Although this might seem as though
it could still undergo hydrogen fusion for another 90 billion years, this is
not the case. Remember that there are immense pressures at the core of stars,
and it is only because of these pressures that the fusion can occur -- in a fixed
volume, increased pressure leads to increased heat. Outside of the range of pressures
there is still mostly hydrogen, but it cannot be used because the pressures are
not high enough to initiate fusion.

The helium core is not hot nor dense enough to fuse to create
energy, so the outward pressure is stopped, and gravity takes over again. Gravity
will contract the star, and eventually a shell of hydrogen around the helium
core will become hot enough to fuse H -> He. This shell will produce more
energy than the previous hydrogen core phase did, so the luminosity will rise.
Not all of the energy will escape, though, and it will go into expanding the
star. This expansion will result in a surface temperature drop. The star will
be in the subgiant star, and the cooler surface will have changed from yellow
to orange-red. This cooling is due to the energy spreading over a larger surface
area, so each unit of area receiving less energy.

The helium "ash" from the hydrogen fusion in the
shell will effectively fall onto the core, which will result in the star continuing
to contract to maintain pressure to hold up the star. Once the mass in the core
is approximately 8% of the sun (the Schonberg-Chandrasekhar Limit), the density
will be so great that the core will no longer act as a perfect gas, and it will
become degenerate.

Now the core will be held up by the Pauli Exclusion Principle,
AKA it will be supported by electron degeneracy. This phase will still have the
hydrogen burning shell, but the star's outer layers will continue to expand,
causing it to cool. This begins the star's Red Giant Ascent.

Red Giant Branch

In this phase, H- can form when neutral H takes
on a free electron. Radiation is easily absorbed by H-, and so the
outer layers will have a high opacity. This high opacity and high energy generation
will lead to convection, where the whole outer envelope will become convective
and the material from the core can rise to the surface in a process called "dredge-up".

As the outer layers continue to expand in this red giant phase,
the ionization drops so there are fewer free electrons and fewer H- ions,
leading to an opacity drop. The luminosity will still rise due to the core contracting.
In this phase, the luminosity is approximately 100 times what it was during the
main part of its life, the radius between 30-100 times, and the effective temperature
will be approximately 60%. Examples of this are Arcturus and Aldebaran.

In
the core, the temperature continues to rise. When it approaches 100,000,000 K
(180,000,000 °F), helium will begin to fuse into carbon in the triple alpha
process. However, since the core is degenerate, when the temperature rises, the
pressure does not, for degenerate pressure is only a function of the density
(right). Therefore, the core cannot expand and cool, so the energy raises the
temperature, which raises the energy which raises the temperature, etc. When
this actually happens, 1011 times the luminosity of the star during
its main sequence life will be released in a few seconds in what is known as
the Helium Core Flash.

Horizontal Branch

None
of the energy from the helium core flash will make it out of the star. It will
act to revert the core back to an ideal gas state, and expand it. Thus, the star
will have a helium burning core, a hydrogen burning shell which will provide
most of the luminosity, and a large expanding envelope of outer atmosphere. The
star will now become a Horizontal Branch Star, for as the core expands, it cools
and the energy generation in the hydrogen burning shell will drop; so the luminosity
decreases, the star will shrink, and the surface temperature rise.

During the course of the horizontal giant branch, a carbon
and oxygen ash core will begin to build up. In a star such as the sun, carbon
fusion cannot occur because the temperature and density are too low. Thus, it
will contract and heat, heating the layers outside the core. This will cause
the new helium shell to start to fuse, and the star will begin to expand again.
This repeats the previous process where there is more energy, a higher opacity,
convection and a dredge-up phase. The again-expanding photosphere and higher
luminosity combine to move the sun into the Asymptotic Giant Branch (AGB), AKA
a red supergiant (cross section to the right).

Asymptotic Giant Branch (AGB)

In the AGB phase, the star will undergo periodic instabilities.
One cause of this are helium shell flashes. This comes from the inert helium
shell continually having mass added to it from the hydrogen shell. Although it
will be in a partially degenerate state, when the mass gets too high, it will "ignite" in
a flash similar to the first one. This will cause it to drop in luminosity and
contract in size, repeating on a timescale of approximately 100,000 years. Instabilities
in the outer envelope can cause AGB stars to pulsate on periods of several hundred
days. Mass is lost during this phase at a rate of approximately 0.01% the mass
of the sun per year in a process that is not well understood.

In
the AGB phase, the outer layers of the star will be greatly extended and will
not be strongly bound. Mass loss, pulsations, and a low binding energy of the
outer layers can cause them to be released from the star, turning this phase
into the Planetary Nebula.

Planetary Nebula

As the outer layers expand, their density will drop, and would
allow future civilizations to view the hot carbon/oxygen (C/O) core that will
be left behind. The C/O core will initially be hot at 100,000 K (180,000 °F).
However, it will be dead, with no nuclear reactions to power it. It will be a
white dwarf. A current example of a white dwarf is the star Sirius B.

The outer layers that will form the planetary nebula will shine
and become visible to outside observers, as is the nebula to the left of IC 418
(AKA The Spirograph Nebula), as taken by the Hubble
Space Telescope. Typically, planetary nebulae are around 0.3 parsecs (1 light-year),
expand at a rate of 10-30 km/s, and last only 10,000 years. For more information
about this nebula, see the Hubble Heritage Archive
- 2000.

When the star runs out of heat, it will be a huge, black, chunk
of carbon and oxygen floating in space. It will be called a black
dwarf - a dead star.

Dwarf Death

Red dwarfs are the only active (undergoing hydrogen fusion)
type of dwarf (other types are brown, white,
and black). Red dwarfs range between
1/3 and 1/12 the sun's mass, and shine only 1/100 to 1/1,000,000 as brightly.
Proxima Centauri, Earth's closest extrasolar star, is a red dwarf 1/5 the size
of the sun, and if it were to trade places with the sun, it would shine on Earth
only 1/10 as much as the sun currently does on Pluto.

Red dwarfs, because of their small size, undergo fusion much
less quickly than a solar mass star. Therefore, they use up their supply of hydrogen
much less quickly than a main sequence star, and can live for more than 100 trillion
years.

When they die, they simply wink out of existence, for they
do not have enough pressure to fuse helium. Thus, they simply grow dimmer and
cooler as they float through the void of space.